Largest Pegmatite Research Articles

Garnet (REE+Y)-bearing mineral association in pegmatite from the Chepino lithotectonic Unit, Western Rhodopes

Large pegmatite body/bodies are outcropping in the “Suhata Reka” quarry, near the town of Rakitovo. Their bulk mineral composition with dominant K-feldspar (40%), plagioclase (30%), quartz (25%) and mica (≤ 5%) is complemented by (REE+Y)-bearing association of primary and secondary accessories. Zircon, monazite, xenotime, scarce apatite and noticeable amount of garnet are among the pegmatite primary minerals. The cm-sized almandine-spessartine crystals contain numerous fractures which host secondary (REE+Y)-bearing phosphate (xenotime, cheralite) and silicate minerals (e.g. REE-epidote). Formation of the late-stage REE-mineralization in garnet is due to its hydrothermal alteration and redistribution of REE+Y in the secondary phases.

Formation of granitic pegmatites during orogenies: Indications from a case study of the pegmatites in China

Granitic pegmatites are a type of igneous rock with very coarse-to gigantic-sized textures, and are the primary source of rare metals (e.g., Li, Be, Nb, and Ta) and gem resources. Their formation is closely related to plate movements and crustal growth that occurred during an orogeny. In order to study the formation patterns of pegmatites and their controlling factors during orogeny, we collected the isotopic ages and compositions of granite-pegmatite fields in the ten major pegmatite belts identified in China. Pegmatites in China first formed in the North China Craton at ∼ 2.7 Ga, with the greatest number of mapped pegmatites having formed during the Mesozoic Era. Over geologic time, the pegmatites type tended to change from abyssal pegmatites to muscovite pegmatites to predominantly the LCT-type pegmatites. Pegmatites that formed within a single orogenic cycle usually crystallized during a brief time period. Within an orogenic belt that was subject to multiple orogenies, different pegmatite groups are associated with the orogenies; the pegmatite groups tend to increase in terms of size, numbers and mineralization intensity from the old to the young orogenic cycle. The pegmatite lineage, identified in the orogenic history of China Mainland, indicates that the convergences of micro-blocks in China constrained pegmatite formation in China. Anataxis of the metasediments (i.e., metapelites), as demonstated by their isotopic compositions, during the orogenies produced the granitic-pegmatitic melts. During the Earth’s crust evolution, the increasing maturity of the pegmatite source-rock metasediments promoted the occurrences of large pegmatites with great degrees of rare element mineralization in the Mesozoic Era in the Songpan-Ganze-West Kunlun and Altay orogenic belts, and the common formation of spodumene-subtype pegmatite rather than petalite-subtype pegmatites in the Phanerozoic orogenies in China. The Archean metasediments in the North China Craton, due to their low maturity with few terrigenous components, did not produce large LCT-type pegmatites. Accordingly, recognizing the linkage of metasediments within the orogenies to pegmatite formation can point to potential prospecting areas for new rare metal deposits. Thick metapelites within their structural-metamorphic domes are important markers of pegmatite prediction. In addition, granites that formed within the pegmatite-forming peak periods during orogenies may preferentially produce pegmatites relative to other time periods.

Ore genesis of the Baishawo Be-Li-Nb-Ta deposit in the northeast Hunan Province, south China: Evidence from geological, geochemical, and U-Pb and Re-Os geochronologic data

The Baishawo deposit is a newly discovered large pegmatite type Be-Li-Nb-Ta deposit that located at the NE Hunan province, South China, with estimated reserves of 16,160 t BeO @ 0.084%, 11,280 t Li2O @ 0.85%, and 3450 t (Ta2O5 + Nb2O5) @ 0.201%. The mineralized pegmatites are mainly hosted in the two-mica monzogranite, suggesting a possibly genetic relationship. Mineral assemblages and textures of pegmatites indicate the Baishawo deposit likely belong to LCT-type. The ore-related two-mica monzogranites have high contents of SiO2 and K2O, belong to strongly peraluminous rocks with A/CNK values of 1.13–1.24, and show S-type and highly evolved features. Zircons from the two-mica monzogranite show a εHf(t) range from −14.1 to −8.0 (average −10.1) and molybdenites from pegmatites show Re contents of 7.06–8.98 μg/g, both indicating that these rocks were likely sourced from the crust materials. The Baishawo two-mica monzogranite yields a LA-ICP-MS zircon U-Pb age of 147.5 ± 1.6 Ma, and the molybdenite from the mineralized pegmatites yields an Re-Os isochron age of 140.2 ± 3.3 Ma, which is in consistent with the coltan U-Pb age of 140.7 ± 1.5 Ma, indicating an Early Cretaceous mineralization age. The obtained ages combined with previous studies in the region indicate that the Early Cretaceous (~140 Ma) is an important rare metal mineralization period in South China.

Combined Geoelectrical Approach DC and IP Methods in the Identification of the Mineralized Bodies Parallel to the NE-SW Tectonic Line of Kadei River: Case of Quartz or Pegmatite Gold Bearing Veins of Ngoura Subdivision (East Cameroon)

A geophysical investigation of subsurface structures using the Syscal Junior 48 resistivity-meter was conducted in Ngoura subdivision (East Cameroon) following a combined geoelectrical direct current (DC) approach involving Resistivity and IP methods. This investigation was allowed to collect data on forty-five (45) profiling lines at three acquisition levels (AB = 100 m, MN = 10 m; AB = 200 m, MN = 20 m and AB = 500 m, MN = 50 m) and two electric panels L1 and L4, using the Schlumberger array. Processing, modeling and interpretation of data collected using the Winsev, Res2Dinv and Surfer software helped in highlighting a conductive and strongly mineralized discontinuity in granite formations, which lined up with the NE-SW Kadei tectonic line. It extends beyond 100 m depth over an average width of 600 m. The mineralization associated with this discontinuity is identified by a high concentration of disseminated metalliferous minerals in quartz or pegmatite veins. The mining reconnaissance works in the study area and those of several authors have characterized this anomaly to a lode gold quartz or large pegmatite. The results of this study correlate with geological and tectonic data for the region marked by NE-SW Kadei tectonic line. Therefore, they confirm the reliability of a geoelectrical DC investigation method combining Resistivity and IP to the identification of ore bodies.

SHRIMP U–Pb Zircon geochronology of the Altai No. 3 Pegmatite, NW China, and its implications for the origin and tectonic setting of the pegmatite

The “Altai No. 3 granitic pegmatite” in NW China is the largest pegmatite in the world that has been found in an orogenic belt; it forms one of the largest muscovite deposits in Asia and is most famous for its concentric-ring structure. Although it has been mined for more than 70 years, the age of formation and mineralization of the pegmatite has not been well determined, despite numerous previous geochronological studies. Here we present new results of SHRIMP U–Pb zircon dating. Three weighted mean 206Pb/ 238U ages of 220 ± 9 Ma, 198 ± 7 Ma and 213 ± 6 Ma were obtained for pegmatite zones 1, 5, and 7, respectively. The analyses were made on clear, recrystallized domains and overgrowth rims of large, opaque high-U zircon grains. We interpret the age of 220 ± 9 Ma for zone 1 to approximately represent the initial time of formation for No. 3 pegmatite. An additional date of 213 ± 2 Ma was determined on zircon from a small subordinate vein formed in the late-stage phase of the No. 3 pegmatite. This age set the late age of emplacement of the pegmatite. The time span of 213 to 198 Ma may reflect late stage hydrothermal alteration. In addition, a weighted mean 206Pb/ 238U age of 277 ± 6 Ma was obtained from small, clear, transparent low-U zircon grains which are more probably xenocrysts. Our new age data indicate that the pegmatite has no genetic relationship with the wall granitic rocks (409 Ma), but is likely related to the early Mesozoic (220 Ma) granitic magmatism that was just recognized. The No. 3 pegmatite was formed in an early Mesozoic anorogenic extensional regime. A stable tectonic setting was undoubtedly required for the formation of the large pegmatite.

Geology and structure of the Sundarijal-Melamchi area, central Nepal

In the Sundarijal- Melamchi area, the following lithological units arc identified based on detailed field mapping: the Talamarang Formation (dark grey feldspathic schist and banded gneiss with garnet and kyanite), Gyalthum Formation (thin- to thick­ banded, light grey to grey, laminated quartzite with mica partings and bands of feldspathic schist. garnetiferous schist, and gneiss), Bolde Quartzite (grey to light grey, very thick-banded, massive quartzite with mica parting and schist bands containing sillimanite). Timbu Formation (light grey to dark grey, intensely deformed and folded quartzite, gneiss, and migmatite with abundant sillimanite), and Golphu Formation (grey to dark grey feldspathic schist and banded gneiss with large pegmatite veins) from bottom to top, respectively.
 The Main Central Thrust (MCT) crosses the Indrawati River north of Majhitar and continues towards the Dhand Khola to SE and joins with the Mahabharat Thrust. It is moderately steep and dips essentially due NW. The footwall of the MCT is made up mainly of slate and phyllite and the grade of metamorphism is quite low in comparison with that of the hanging wall. The inverted metamorphism is conspicuous in the hanging wall of the MCT, where at first garnet schist is observed, which passes rapidly upwards to kyanite schist and then to sillimanite-bearing schist, gneiss, and migmatite.
 Augen and banded gneisses invade the upper part of the northern limb and most of the southern limb of the Patibhanjyang Anticline. Apparently, the Sheopuri Injection Gneiss Zone is a continuation of the Kutumsang Gneiss Zone in the north, and the rocks were folded after the emplacement of the injection gneisses.

Uranium-bearing pegmatites of the antsirabe-kitsamby district, madagascar

Madagascar, sixth largest island in the world, is about 75% covered by a deep red laterite mantle explaining its sobriquet “L'île rouge” — the red island. In 1907 a Malagasy geologist discovered euxenite-bearing Tertiary lake sediments near Vinaninkarena, south of Antsirabe in Central Madagascar. Interest in radium stimulated search for primary uranium minerals in the adjacent Precambrian plateau known to contain pegmatites. Prospecting in 1908–1909 identified betafite-euxenite-and samiresite-rich pegmatites near Betafo in the Kitsamby hills and other uraniferous pegmatites were found around Ampangabe, Ambatohasana and Ambatofotsikely. 1919–1924 marked the peak of productive activity, when about 25 pegmatites were exploited: Ambatofotsy, Ambatohasana and Ambatofampykely accounting for over 50 tonnes, chiefly betafite, averaging 12–15% of U 3O 8, or about 50% of Madagascar's total uranium production. The major uraniferous pegmatites are identified by a central massive quartz core in deeply lateritized terrain. In simplest form, they consist of microcline and quartz, with or without accessory biotite, muscovite, black tourmaline, beryl, alamandine and apatite. There are two contrasting sub-types. 1. (1)Pegmatites containing columbium-tantalum-complex, rare-element “rest” or pocket minerals carrying uranium and thorium. Biotite zones contain the predominant mica, with little or no beryl or muscovite. 2. (2)Deposits rich in beryl and muscovite, with biotite subordinate or absent. This paper describes the physical setting, geology and mineralogy of four large pegmatites that accounted for a major portion of Madagascar's betafite-euxenite production, namely Ambotofotsy, Ambatohasana, Ambatolampykely and Vorondola in the Kitsamby district. A map or a section of each deposit is provided.

Age relations of Sveconorwegian granitoid rocks in the Stora Le-Marstrand belt, Orust area, Sweden

Abstract Two generations of Sveconorwegian granitoid rocks are recognized in the Orust-Skaftolandet sector of the Stora Le-Marstrand belt of SW Sweden. The earlier is a suite of large pegmatites containing quartz, plagioclase, K-feldspar, muscovite, minor biotite and garnet. These bodies cut the D3-deformed Orust dykes which have a metamorphic age of 1060 Ma. They are in turn deformed by F4 folds. Rb-Sr ages calculated from three mineral pairs from two of these pegmatite bodies arc close to a mean value of 950 Ma which is interpreted as the emplacement age of the pegmatites. The later generation of Sveconorwegian granitoid is represented by the 891 Ma Bohus granite pluton and associated minor pegmatites. In the islands west of Skaftolandet the Bohus granite contact clearly cuts the Orust dykes and also elements of the post-dyke (Sveconorwegian) deformations, D3 and D4 (and probably D6). The granite contact itself is undeformed although the granite elsewhere exhibits minor shears. These granitoid ages plac...

Archaean and Proterozoic crustal evolution in Lofoten–Vesterålen, N Norway

The region exposes an unusually deep section through the continental crust. Gravity and seismic surveys show a ridge-like NE-SW up-warping of the Moho to within 25 km of the surface. The oldest rocks are migmatitic gneisses of generally intermediate composition, probably largely of supracrustal origin; these were intruded by a granodiorite/granite pluton at about 2600 Ma. Pb isotope data indicate that the migmatites formed c. 2700 Ma ago, and the isotopic systems in some rocks have been disturbed by Proterozoic granulite-facies metamorphism. A Proterozoic supracrustal series is composed dominantly of felsic metavolcanic gneisses but includes marble, quartzite, graphite schist and iron formations. An Rb-Sr whole-rock isochron age of 1830 ±35 Ma is interpreted as the age of a metamorphism which reached intermediate-pressure granulite facies in the western part of the area. An early low-P, high-T event was followed by PT conditions near 900°C and 10 kbar at the metamorphic maximum. This thermal peak was essentially post-tectonic and coincided with the intrusion of gabbro, anorthosite and huge volumes of mangeritic to charnockitic magma between c. 1800 and c. 1700 Ma ago. A swarm of alkali-olivine dolerites was intruded shortly after the mangerites. The Lefdingen granitic batholith gives an Rb-Sr isochron age of 1380 ±80 Ma and may represent remobilized basement rocks. Pelitic schists of the Leknes Group were tectonically emplaced against the mangerites and gneisses, and metamorphosed in amphibolite facies 1140 ±135 Ma ago. Rb-Sr and K-Ar mineral ages suggest that the extensive retrograde metamorphism observed in the older rocks occurred at this time. Small granitic pegmatites were also emplaced during this episode. The Caledonian orogeny is recorded by some K-Ar mineral ages, the development of local W-dipping thrust zones, and the local intrusion of large pegmatites. Lofoten-Vesteralen was probably part of the Baltic plate during the Caledonian orogeny, but escaped deformation because it lay at a high tectonic level and consisted largely of massive granulite-facies rocks.

Uranium and lead gain of detrital zircon studied by isotopic analyses and fission-track mapping

Besides Pb and U loss and mixing of crystals of different age, U gain is considered a possible cause of discordant U-Pb ages of zircons. However, whether U gain without new zircon growth occurs in nature had not been proven, so far. In order to test this possibility, two detrital zircon populations were studied for which the absence of later zircon overgrowth after deposition could be demonstrated. The samples were separated from a metaquartzite near a large pegmatite body and from metaquartzite inclusions found in the pegmatite (Martell Valley, Italian Alps). The distribution of neutron-induced fission tracks reveals distinct accumulation of U in the rims of more than 90% of the zircon grains of the inclusions (total U in the crystals: 540–850 ppm), whereas in the country rock only some of the grains show similar but weaker patterns (total U: 155–320 ppm). From the isotopic data and from additional U-Pb and Rb-Sr analyses of minerals and whole-rock samples of the pegmatite, the marginal accumulation and the higher concentration of U in the zircon grains of the inclusions are interpreted as the result of episodic U gain during the intrusion of the pegmatite and/or during a later metamorphism. From the concentration levels of common Pb, an addition of Pb - and possibly other elements - to the zircon grains is inferred.

Distribution of Ca, Sr and Ba in southern Norwegian pre-Cambrian alkali feldspars

Ca, Sr and Ba have been determined spectrochemically in alkali feldspars from gneisses, augengneisses, anatexe granites, diapire granites and small and large pegmatites from the pre-Cambrian basement rocks of southern Norway. Ca decreases in feldspars from large pegmatites, but not to the same extent as Sr and Ba. It is shown that Sr readily substitutes in K-feldspar. A close association of Ba and Sr was found. With increasing fractionation the Ba/Sr ratio decreases and is lowest in the large pegmatites, as are the absolute amounts of Sr and Ba. The behaviour of Ca, Sr and Ba is explained mainly by the relative bond strengths; Ba which forms the bond with most ionic character is preferentially accepted.

Distribution of Li, Na, K, Rb, Cs, Pb and Tl in southern Norwegian pre-Cambrian alkali feldspars

The concentrations of Li, Rb, Cs, Pb and Tl have been determined spectrographically and Na and K by flame photometer in eighty-eight feldspars from the pre-Cambrian basement rocks of southern Norway. Bock types include gneisses, augengneisses, anatexe granites, diapire (post-orogenic) granites and small and large pegmatites. Analytical procedures are described. Normal K/Rb ratios are found except for those from some large pegmatite feldspars which are enriched in Rb, and feldspars from arendalites which are relatively impoverished. The latter effect is shown to be related to feldspar structure and bulk composition. Changes in the K/Rb ratio are traced with increasing differentiation in the Fevig diapire granite. Feldspare from small pegmatites are similar in composition to the country rock. Those from large pegmatites fall into three compositional groups. The trace element content of the feldspars seems in general to reflect the differences in geological conditions under which they were formed.

Alkali elements in potash feldspar from the pre-Cambrian of Southern Norway

Li, Na, K, Rb, and Cs have been determined in 50 alkali feldspars from the pre-Cambrian of South Norway. The rock types range from gneisses, augengneisses, gneiss granites, granites to aplites and pegmatites. Rubidium enrichment occurs in some diapire (post-orogenic) granites and in some of the large pegmatites from the Telemark formation. Normal K/Rb ratios are found for the large pegmatites from Kragerø and Arendal as well as the gneisses, etc. Enrichment in rubidium was found in all feldspars examined from east of the Oslo fiord.

Comparison of radiogenic helium and lead in zircon

A direct comparison of helium and radiogenic lead in nonmetamict zircon crystals has indicated a 25 per cent discrepancy between these radiogenic constituents. It appears that the amount of helium is only three-quarters of the amount that should be present according to the content of radiogenic lead. Approximately the same proportion was found in zircon having a wide range in age, activity, and total alpha irradiation, and in large pegmatite zircon crystals, as well as in microscopic crystals from igneous rocks.